Polymer Powder Composition For Additive Manufacturing

ABSTRACT

A powder composition suitable for use in selective laser sintering for printing an object. The powder composition includes a first fraction having a polyetherketoneketone (PEKK) powder having a plurality of particles. The powder composition includes a second fraction having a plurality of siloxane particles that is dry blended with the PEKK powder prior to selective laser sintering of the powder. In some embodiments, the powder composition includes a third fraction of carbon fiber. The powder composition when used in selective laser sintering results in parts with more consistent results and improves process economics.

TECHNICAL FIELD

The present disclosure generally relates to additive manufacturingtechnology and techniques, and more specifically relates to a polyetherether ketone (“PEKK”) powder composition for use in selective lasersintering (“SLS” or “LS”), a method for preparing the powdercomposition, and a method for additively manufacturing an object usingthe PEKK powder composition.

BACKGROUND

It is known to use additive manufacturing technology and techniques,together with polymer powders, to manufacture high-performance productshaving applications in various industries (e.g., aerospace, industrial,medical, etc.).

SLS is an additive manufacturing technique that uses a laser to fusesmall particles of plastic, metal (direct metal laser sintering),ceramic, or glass powders into a mass having a desired three-dimensional(3-D) shape. The laser selectively fuses the powder material by scanningcross-sectional layers generated from a 3-D digital description of thedesired object onto the top layer of a bed of the powder material. Aftera cross-sectional layer is scanned, the powder bed is lowered byone-layer thickness in a z-axis direction, a new top layer of powdermaterial is applied to the powder bed, and the powder bed is rescanned.This process is repeated until the object is completed. When completed,the object is formed in a “cake” of unfused powder material. The formedobject is extracted from the cake. The powder material from the cake canbe recovered, sieved, and used in a subsequent SLS process.

Polyaryletherketones (“PAEK”) are of interest in the SLS process becauseparts that have been manufactured from PAEK powder or PAEK granulatesare characterized by a low flammability, a good biocompatibility, and ahigh resistance against hydrolysis and radiation. The thermal resistanceat elevated temperatures as well as the chemical resistancedistinguishes PAEK powders from ordinary plastic powders. A PAEK powdermay be a powder from the group consisting of polyetheretherketone(“PEEK”), polyetherketoneketone (“PEKK”), polyetherketone (“PEK”),polyetheretherketoneketone (“PEEKK”) or polyetherketoneetherketoneketone(“PEKEKK”).

PEKK powders are of particular interest in the SLS process becauseobjects that have been manufactured from PEKK powders via SLS havedemonstrated not only the above characteristics but also superiorstrength relative to other PAEK materials.

PEKK powders are unique in the SLS technique because unused PEKK powdercan be recycled in subsequent SLS processes and the resultant piecesexhibit increased strength as compared to similar parts made with virginpowder. After an SLS build, the mass yield from the built part relativeto the unsintered powder is typically less than 20% of the powdermaterial used in the LS process.

After the parts are removed from the cake bed, the remaining PEKKmaterial is referred to as used PEKK material or recycled PEKK material.This material is referred to as used or recycled because it has beenused at least once in the SLS process. In other words, this material hasbeen raised to the bed temperature and added to the bed in a layer-wisefashion. Material adjacent to the used material was sintered in theinitial SLS process.

After the parts are removed from the cake, the PEKK powder forming thecake is recycled for subsequent use in the SLS process. Sieving of thecake is performed to restore common size to the recovered cakestructure, which is typically lumpy. The sieve size may be similar tothe original powder or the sieve size may be different than the originalpowder. In the process described, it is preferred that the sieve sizefalls in the 20-200 micron range. It is possible to blend batches ofused sieved PEKK powder. However, it is preferred that batches of usedsieved PEKK powder that are blended have similar thermodynamicproperties. The use of DSC, FTIR, and other analytical methods may beused to determine which batches of used sieved PEKK powder can be mixed.A test build can be used to validate analytical results.

The Applicant is the owner of U.S. Patent Publication No. US 20130217838for a Method for Processing PAEK and Articles Manufactured from the Samethe contents of which are incorporated herein by reference. As set forthin that disclosure, the SLS powder may be subject to multiple iterationsof recycle. The term virgin powder in the context of SLS recyclingrefers to a SLS powder that has not been subjected to ambient chamberconditions during a SLS build. The term first recycle or Cake A refersto a batch of powder that has been previously exposed to ambient chamberconditions in one SLS build. The term second recycle or Cake B refers toa batch of powder that has been previously exposed to ambient chamberconditions in two SLS builds. Cake C has been exposed to three builds,and so forth.

A disadvantage of known recycle methods for PEKK powder is that it isnot possible to reliably build parts from Cake B powder. As a result,significant amounts of unsintered powder is wasted in the SLS process,resulting in significant expense.

Another disadvantage of known methods and powder compositions is thatattempted SLS of Cake B powder PEKK causes unwanted variations in thethickness of the built part, particularly in out-of-plane surfacesresulting in nonconforming parts that are not acceptable to customers.

A disadvantage of performing SLS on powder compositions with Cake B PEKKis that it is difficult to build objects when Cake B PEKK is included inthe feedstock because it inhibits the application of powder in the SLSmachine. For example, the Cake B may cause pilling, sticking, and otherforms of fouling in steps of the SLS process in which smooth flowingpowder are required. Therefore, it is understood that it is not possibleto operate the SLS machine to build parts using Cake B PEKK.

Another disadvantage of the SLS process using Cake B is that it resultsin parts that difficult to remove from the powder bed and clean relativeto parts made from virgin powder or Cake A powder.

SUMMARY

The needs set forth herein as well as further and other needs andadvantages are addressed by the present teachings, which illustratesolutions and advantages described below.

It is an objective of the present teachings to remedy the abovedrawbacks and issues associated with prior art selective laser sinteringmethods and powder compositions.

The present invention resides in one aspect in a powder compositionsuitable for use in selective laser sintering for printing athree-dimensional object. The powder composition includes a firstfraction comprising a polyetherketoneketone (PEKK) powder having aplurality of particles. The plurality of particles have a mean diameterbetween 30 μm to 90 μm. The powder composition includes a secondfraction having a siloxane powder having a plurality of particles.

In yet a further embodiment of the present invention, the powdercomposition comprises a third fraction comprising carbon fiber, thethird fraction is between 5% and 25% of the composition by weight.

In yet a further embodiment of the present invention, the PEKK powderhas been previously used in a first SLS build having a bed temperatureabove 250° C.

In yet a further embodiment of the present invention, the PEKK powderhas been previously used in a second SLS build having a bed temperatureabove 250° C.

In yet a further embodiment of the present invention, the secondfraction is between 0.0% and 5.0% of the composition by weight.

In yet a further embodiment of the present invention, the secondfraction is between 0.0% and 2.5% of the composition by weight.

In yet a further embodiment of the present invention, the secondfraction is between 0.5% and 1.5% of the composition by weight.

In yet a further embodiment of the present invention, the secondfraction is 1.0% of the composition by weight.

In yet a further embodiment of the present invention, the third fractionis 15% of the composition by weight.

In yet a further embodiment of the present invention, the plurality ofsiloxane particles have a mean diameter between 30 μm to 60 μm.

In yet a further embodiment of the present invention, the plurality ofsiloxane particles have a mean diameter between 40 μm to 50 μm.

The present invention resides in another aspect in a method for alayer-wise manufacturing of a three-dimensional object from the powdercomposition described above. The method includes the steps of applying alayer of the powder composition on a bed of a laser sintering machine,and then solidifying selected points of the applied layer of powder byirradiation. These steps are successively repeated until all crosssections of a three-dimensional object are solidified.

In yet a further embodiment of the present invention, the object formedhas a strain to failure of at least 2.0%.

In yet a further embodiment of the present invention, the object formedhas a strain to failure greater than a strain to failure of an objectmade from a PEKK powder composition excluding siloxane powder.

The present invention resides in another aspect in a method of preparinga powder composition suitable for use in laser sintering for printing athree-dimensional object. The method includes the steps of providing apolyetherketoneketone (PEKK) powder having a plurality of particles,providing a siloxane powder having a plurality of particles, and mixingthe PEKK powder with the siloxane powder to obtain a powder compositionsuitable for use in selective laser sintering.

In yet a further embodiment of the present invention, the mixing of thePEKK powder and the siloxane powder is performed under dry conditions.

In yet a further embodiment of the present invention, the methodincludes the step of mixing the carbon fibers into the powdercomposition.

In yet a further embodiment of the present invention, the PEKK powderhas been previously used in a first SLS build having a bed temperatureabove 250° C.

In yet a further embodiment of the present invention, the PEKK powderhas been previously used in a second SLS build having a bed temperatureabove 250° C.

In yet a further embodiment of the present invention, the siloxanepowder is between 0.0% and 5.0% of the composition by weight.

In yet a further embodiment of the present invention, the siloxanepowder is between 0.0% and 2.0% of the composition by weight.

In yet a further embodiment of the present invention, the secondfraction is between 0.5% and 1.5% of the composition by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a LS machine in accordance with one embodiment of thepresent invention.

FIG. 2 is an image of a portion of a part built from Cake B PEKK andcarbon fiber using SLS.

FIG. 3 is an image showing a magnified view of a cross section of a partbuilt from CAKE B and carbon fiber.

FIG. 4 is an image showing a magnified view of a cross section of a partbuilt from CAKE B, carbon fiber, and siloxane.

FIG. 5 is chart showing temperature data for powder compositions.

DETAILED DESCRIPTION

The present disclosure describes aspects of the present invention withreference to the exemplary embodiments illustrated in the drawings;however, aspects of the present invention are not limited to theexemplary embodiments illustrated in the drawings. It will be apparentto those of ordinary skill in the art that aspects of the presentinvention include many more embodiments. Accordingly, aspects of thepresent invention are not to be restricted in light of the exemplaryembodiments illustrated in the drawings. It will also be apparent tothose of ordinary skill in the art that variations and modifications canbe made without departing from the true scope of the present disclosure.For example, in some instances, one or more features disclosed inconnection with one embodiment can be used alone or in combination withone or more features of one or more other embodiments.

The present invention is especially useful for preparing polymer powdersfor laser sintering. One such class of polymer powders isPolyaryletherketones (“PAEK”) polymers. PAEKs are of interest in the SLSprocess because parts that have been manufactured from PAEK powder orPAEK granulates are characterized by a low flammability, a goodbiocompatibility, and a high resistance against hydrolysis andradiation. The thermal resistance at elevated temperatures as well asthe chemical resistance distinguishes PAEK powders from ordinary plasticpowders. A PAEK polymer powder may be a powder from the group consistingof polyetheretherketone (“PEEK”), polyetherketoneketone (“PEKK”),polyetherketone (“PEK”), polyetheretherketoneketone (“PEEKK”) orpolyetherketoneetherketoneketone (“PEKEKK”).

PEKKs are well-known in the art and can be prepared using any suitablepolymerization technique, including the methods described in thefollowing patents, each of which is incorporated herein by reference inits entirety for all purposes: U.S. Pat. Nos. 3,065,205; 3,441,538;3,442,857; 3,516,966; 4,704,448; 4,816,556; and 6,177,518. PEKK polymersdiffer from the general class of PAEK polymers in that they ofteninclude, as repeating units, two different isomeric forms ofketone-ketone. These repeating units can be represented by the followingFormulas I and II:

-A-C(═O)—B—C(═O)—  Formula I

-A-C(═O)—D-C(═O)—  Formula II

where A is a p,p′-Ph—O—Ph-group, Ph is a phenylene radical, B isp-phenylene, and D is m-phenylene. The Formula I:Formula II isomerratio, commonly referred to as the T:I ratio, in the PEKK is selected soas to vary the total crystallinity of the polymer. The T:I ratio iscommonly varied from 50:50 to 90:10, and in some embodiments 60:40 to80:20. A higher T:I ratio such as, 80:20, provides a higher degree ofcrystallinity as compared to a lower T:I ratio, such as 60:40.

The crystal structure, polymorphism, and morphology of homopolymers ofPEKK have been studied and have been reported in, for example, Cheng, Z.D. et al, “Polymorphism and crystal structure identification inpoly(aryl ether ketone ketone)s,” Macromol. Chem. Phys. 197, 185-213(1996), the disclosure of which is hereby incorporated by reference inits entirety. This article studied PEKK homopolymers having allpara-phenylene linkages [PEKK(T)], one meta-phenylene linkage [PEKK(I)],or alternating T and I isomers [PEKK(T/I)]. PEKK(T) and PEKK(T/I) showcrystalline polymorphism depending upon the crystallization conditionsand methods.

In PEKK(T), two crystalline forms, forms I and II, are observed. Form Ican be produced when samples are crystallized from melting at lowsupercooling, while Form II is typically found via solvent-inducedcrystallization or by cold-crystallization from the glassy state atrelatively high supercooling. PEKK(I) possesses only one crystal unitcell which belongs to the same category as the Form I structure inPEKK(T). The c-axis dimension of the unit cell has been determined asthree phenylenes having a zig-zag conformation, with the meta-phenylenelying on the backbone plane. PEKK(T/I) shows crystalline forms I and II(as in the case of PEKK(T)) and also shows, under certain conditions, aform III.

Suitable PEKKs are available from several commercial sources undervarious brand names. For example, polyetherketoneketones are sold underthe brand name OXPEKK® polymers by Oxford Performance Materials, SouthWindsor, Conn. Polyetherketoneketone polymers are also manufactured andsupplied by Arkema. In addition to using polymers with a specific T:Iratio, mixtures of polyetherketoneketones may be employed.

The powders used in these applications are produced by a variety ofprocesses such as grinding, air milling, spray drying, freeze-drying, ordirect melt processing to fine powders. The heat treatment can beaccomplished before or after the powders are produced, but if treatedprior to forming the powders, the temperature of the powder formingprocess must be regulated to not significantly reduce the meltingtemperature or the quantity of the crystallinity formed in the heattreatment process.

In the method of preparing the powders in accordance with the presentinvention, a raw PEKK flake is provided. The raw PEKK flake iscommercially available from companies such as Arkema, Inc. of King ofPrussia, Pa., and Cytec Industries Inc. of Woodland Park, N.J. The rawPEKK flake is typically swilled from a chemical reactor and then washed.The raw PEKK flake is a non-powder material. That is, the raw PEKK flakeis not in the form of a powder that can be used in the LS. The raw PEKKflake is in the form of irregularly-shaped particles (e.g., particlesthat are vaguely round, elongated, flat, etc.) and has an appearancesimilar to that of Rice Krispies® cereal. The irregularly-shapedparticles of the raw PEKK flake have grain sizes that are orders ofmagnitude larger than 150 μm, for example. The remainder of the raw PEKKflake can be in the form of a gel or gel-like form caused by an amountof liquid solvent remaining from the process of producing the raw PEKK.

After the step of providing the raw PEKK flake, a heat treatment step isoptionally performed prior to grinding. The heat-treatment process isthe subject of U.S. patent application Ser. No. 15/872,478 filed on Jan.16, 2018 by Hexcel Corporation and titled “Polymer Powder and Method ofUsing the Same.” The disclosure of that reference is hereby incorporatedby reference. After the heat-treating step, a cooling step is performedthat involves powering-off the convection oven and allowing the raw PEKKto cool naturally.

After the cooling step, a grinding or milling step is performed thatinvolves grinding the raw PEKK flake to form what will hereinafter bereferred to as the “PEKK powder.” The grinding step can be performedusing known grinding techniques performed by companies such as Aveka,Inc. of Woodbury, Minn. Upon completion of the grinding step, theparticles of the PEKK powder are significantly smaller (i.e., severaldegrees of magnitude smaller) than the particles of the raw PEKK. Theparticles of the PEKK powder are more consistent and regular in shape ascompared to the particles of the raw PEKK; however, the particles of thePEKK powder are still irregularly-shaped in comparison to thespherical-shaped particles.

A person of ordinary skill in the art and familiar with this disclosurewill understand that the grinding may also be referred to aspulverization, milling, or jet milling. In addition, a person ofordinary skill in the art and familiar with this disclosure willunderstand that it may also be employed with other polymer powders,including those in the PAEK family.

The particles in accordance with the present invention were ground fromthe flake. A mill is used that incorporates dense phase micronizationusing turbulent, free jets in combination with high efficiencycentrifugal air classification within a common housing. This providescomminution by high probability of particle-on-particle impact forbreakage and a high degree of particle dispersion for separation. Theresultant particles are non-spherical and substantially angular. This isa result of the jet milling process that performs particle comminutionvia particle-on-particle impact. The substantial non-spherical PEKKparticles perform better in the LS process. For example, thenon-spherical particles are more easily distributed on the bed table forthe LS process and the non-spherical particles result in substantiallystronger parts, particularly in the out-of-plane axis. The increasedperformance of non-spherical particles is due in part to the increasedability for larger and smaller particles to pack together enhancing thestrength of the laser fusion.

The raw PEKK flake is ground into a PEKK powder comprising a pluralityof PEKK particles. The PEKK particles range in size from less than 10 μmto about 200 μm. A person of ordinary skill in the art and familiar withthis disclosure will understand that the particle size range will varybased on the type of polymer being milled and the specific parameters ofthe milling process.

After the milling, an air classification method may be used to separatefine particles from the milled PEKK powder. It is known in the art thatit is necessary to reduce or eliminate particles having a diameter belowa cut-off point, for example 30 μm, as it has been found that particlesin this range prevent use of the powder in the LS process. For example,International Patent Application WO2014100320 discloses such a methodfor preparing powders for use in selective laser sintering.

After the grinding step, another optional processing step is performedthat involves adding an amount of carbon fiber to the PEKK powder. Themixing process is the subject of US Publication No. US20180201783published on Jul. 17, 2018 by Hexcel Corporation and titled “PolymerPowder and Method of Preparing the Same.” The disclosure of thatreference is hereby incorporated by reference.

In accordance with one embodiment of the present invention carbon fiberavailable from Hexcel Corporation of Stamford, Conn., USA and sold underthe brand name HEXTOW® AS4 is employed. The carbon fiber is acontinuous, high strength, high strain, PAN based fiber. In thisembodiment, the carbon fiber has a filament diameter of approximately7.1 μm and is wound on a cardboard tube. It should be understood to aperson having ordinary skill in the art that different types and brandsof carbon fibers may be employed, and that the present invention is notspecifically limited in this regard. The carbon fiber is milled prior toincorporation into the PEKK powder to achieve the desired carbon fiberlength as determined by the average L50. The carbon fiber is milled by amiller such as E&L Enterprises Inc. in Oakdale, Tenn., USA. For example,in one embodiment of the present invention, the mean carbon length, L50,is 77 μm. The minimum length measured is 38.15 μm, the maximum lengthmeasured is 453 μm, and the standard deviation is 42.09 μm.

A powder composition suitable for use in a selective laser sintering forprinting a three-dimensional object is prepared combining a PEKK powderwith the carbon fiber. In some embodiments of the present invention thecomposition includes 85% by weight of PEKK powder and 15% by weightcarbon fiber. It yet other embodiments of the present invention, theamount of carbon fiber is varied relative to the polymer powder toachieve composition for SLS. In some embodiments of the presentinvention, one or more additives are added to the matrix to affect theproperties of the SLS composition, for example, during the printingprocess or in the printed article. It will be understood to a person ofordinary skill in the art and familiar with this invention, that theratio of carbon to polymer may vary and the above examples are providedfor illustration purposes.

According to one embodiment of the present invention, in reference toFIG. 1, a LS system 10 in accordance with the present invention isillustrated. The system 10 includes a first chamber 20 having anactuatable piston 24 deposed therein. A bed 22 is deposed at an end ofthe piston 24. It should be understood that the term bed may refer tothe physical structure supported on the piston or the uppermost layer ofpowder deposed thereon.

The temperature of the bed 22 can be variably controlled via acontroller 60 in communication with heating elements (not shown) in oraround the bed 22. Furthermore, the LS system 10 according to theinvention may include a heating device (not shown) above the bed 22,which preheats a newly applied powder layer up to a working temperaturebelow a temperature at which the solidification of the powder materialoccurs. The heating device may be a radiative heating device (e.g., oneor more radiant heaters) which can introduce heat energy into the newlyapplied powder layer in a large area by emitting electromagneticradiation.

A second chamber 30 is adjacent to the first chamber 20. The secondchamber 30 includes a table surface 32 disposed on an end of a piston 34deposed therein. A powder 36 for use in the LS system 10 is stored inthe second chamber 30 prior to the sintering step. It will be understoodto a person of ordinary skill in the art and familiar with thisdisclosure that while a specific embodiment of a LS system is disclosed,the present invention is not limited thereto, and different known LSsystems may be employed in the practice of the present invention.

During operation of the LS system 10, a spreader 40 translates across atop surface of the first chamber 20, evenly distributing a layer ofpowder 36 across onto either the top surface of the bed 22 or thematerial previously deposed on the bed 22. The LS system 10 preheats thepowder material 36 deposed on the bed 22 to a temperature proximate to amelting point of the powder. Typically, a layer of powder is spread tohave a thickness of 125 μm, however the thickness of the layer of powdercan be increased or decreased depending on the specific LS process andwithin the limits of the LS system.

A laser 50 and a scanning device 54 are deposed above the bed 22. Thelaser 50 transmits a beam 52 to the scanner 54, which then distributes alaser beam 56 across the layer of powder 36 deposed on the bed 22 inaccordance with build data. The laser selectively fuses powder materialby scanning cross-sections generated from a three-dimensional digitaldescription of the part on the surface of the bed having a layer of thepowder material deposed thereon. The laser 50 and the scanner 54 are incommunication with the controller 60. After a cross-section is scanned,the bed 22 is lowered by one layer thickness (illustrated by thedownward arrow), a new layer of powdered material is deposed on the bed22 via the spreader 40, and the bed 22 is rescanned by the laser. Thisprocess is repeated until a build 28 is completed. During this process,the piston 34 in the second chamber is incrementally raised (illustratedby the upward arrow) to ensure that there is a sufficient supply ofpowder 36.

The inventors have identified a disadvantage SLS of PEKK powder and SLSof PEKK powder with carbon fiber additive is that it is not possible toprint parts from Cake B PEKK. The first problem is that Cake B performsvery poorly during the powder coating process in the SLS machinerelative to virgin powder and Cake A. The Cake B PEKK powder tends toclump together and foul the coating process such that it causes many SLSbuilds, which can last between several hours and several days dependingon the depth of the build, to fail. As a result of these failures it isunderstood that Cake B PEKK cannot be used to print parts on acommercial scale.

In some circumstances it is possible to avoid the powder applicationissues with Cake B PEKK and print parts using the SLS process. In thesecircumstances, the SLS process remains commercially unfeasible becausethe removal of the sintered parts from the powder bed is substantiallymore time consuming and difficult because the unsintered powder isdifficult to remove from the built parts relative to parts made fromCake A and virgin. The unsintered powder tends to clump to the part andinhibit removal of the part from the powder bed rendering use of Cake BPEKK unfeasible.

Another disadvantage of printing SLS Cake B PEKK, to the extent it ispossible, is that it results in parts with significant deviations inpart thickness between the built part in the design file inputted intothe SLS machine. The thickness deviations are visible in out-of-planesurfaces in the form of striping on the surface of parts. This issometimes referred to as striping or tiger striping. In reference toFIG. 2, an image showing a portion of a part built from Cake B PEKK with15% carbon fiber using SLS is shown. In this image, the striping isreadily visible on the surface of the built part.

The striping in the surface of the part precludes use of PEKK B forseveral reasons. First, the striping negatively affects the aestheticsof the constructed part, thereby reducing the desirability of the SLSprinted part by customers. Second, the deviations thickness are to suchan extent that the parts are consistently rejected by customers,particularly in aerospace and other areas that require high precisionmanufacturing. For example, a customer may accept parts that are within−0.010″/+0.020″ of the specified drawing dimension. Companies such asthe Applicant can routinely print SLS parts using virgin PEKK and Cake APEKK that meet this requirement. However, it is not possible to reliablyprint SLS parts from Cake B that meet this requirement.

The inventors have observed through significant research, analysis andreview that as the PEKK powder is subject to ambient temperatures in theSLS chamber, typically less then 20 C below the highest melting point ofthe PEKK, the acceptable window for the ambient chamber temperaturenarrows. A person of skill in the art and familiar with this disclosurewill understand that the ambient chamber temperature is also referred toas bed temperature. In the SLS machine used by the Applicant, the EOSPSINT800, the ambient chamber temperature or bed temperature is measuredby an infrared sensor that measures a fixed point on the bed surface. Abed temperature setpoint is maintained via a controlled system thatadjusts radiant heaters above the bed in response to the temperaturedetected by the infrared sensor.

With virgin PEKK powder with 15% fiber carbon, the inventors haveobserved an approximately 10° C. acceptable window in bed temp setpoint. For example, the bed temperature setpoint is determined to be270° C. based on an analytical method. The inventors have discoveredthat it is possible to operate the SLS machine using this powder at thedetermined bed temperature of 270° C.+ or −5° C. without observingfouling issues in the powder application, powder removal, or partqualification. The SLS machine controls the bed temperature setpoint towithin +−3 C of the setpoint. As a result, there is a high certaintythat the bed temperature is maintained within the acceptable window forthe virgin PEKK+15% carbon fiber.

During the first run in the SLS chamber, the unsintered powder isexposed to the ambient bed temperature during the duration of the build.After the build, the exposed unsintered powder is referred to as Cake A.Cake A can then be used in a subsequent SLS build. First, the bedtemperature setpoint for the Cake A is determined using the analyticalmethod. The inventors have discovered bed temperature exposure historyof Cake A narrows the acceptable bed temperature operating window forsubsequent SLS builds to about 5 or 6 C. Therefore, it is still highlylikely that the SLS machine can maintain the required bed temperaturefor Cake A.

With Cake B, the acceptable window is 1 or 2° C. Therefore, with amachine that runs +−3° C., it is not possible to run the machineconsistently within the required window.

The inventors have overcome this problem by employing the siloxane resinadditive. It improves flow of the heat degraded PEKK powder in themachine. The inventors have discovered that by dry blending siloxanepowder with the Cake B it is possible to repeatedly print parts usingSLS without variation in part thickness, or without issues associatedwith powder application or part removal. The siloxane agent alsoimproved the toughness of the material unexpectedly. The benefits ofsiloxane additive and preferred amounts are discussed further below.

Examples

The tables below include data associated with three different PEKKpowders. Each powder comprises Cake B PEKK that was prepared inaccordance with the method described below. Test specimens were printedin both in-plane and out-of-plane directions using the prepared powdersvia an SLS machine and tested in accordance to ASTM D6272 (flexural) andASTM D638 (tensile).

TABLE 1 Composition (% wt.) Example Batch Cake B Carbon Siloxane T_(M)(° C.) Example 1 1734 85 15 0 306.98 Example 2 1870 84.15 14.85 1 307.60Example 3 2116 83.3 14.7 2 307.06

In reference to Table 1, data regarding the tested powder compositionsis provided. Example 1 refers to powder batch 1734. The batch includes85% Cake B, or PEKK powder that had been exposed to ambient chamberstemperatures during two SLS builds, by weight. Example 1 furtherincludes 15% carbon fiber by weight. It does not include any siloxane.The powder composition of example 1 has a melting temperature of around307.0° C.

Example 2 refers to powder batch 1870. The powder batch includes 84.15%Cake B by weight. The Cake B PEKK powder is sourced from the same Cake Bpowder used to prepare the powder composition of Example 1. Example 2includes 14.85% carbon fiber by weight. It includes 1% siloxane byweight. The powder composition of example 2 has a melting temperature ofaround 307.6° C.

Example 3 refers to powder batch 2116. The powder batch includes 83.3%Cake B by weight. The Cake B PEKK powder is sourced from the same Cake Bpowder used to prepare the powder composition of Example 1. Example 3includes 14.7% carbon fiber by weight. It includes 2% siloxane byweight. The powder composition of example 3 has a melting temperature ofaround 307.1° C.

In each of the tested powder compositions, the raw PEKK flake had a T:Iratio of 60:40 and was supplied by either Arkema, Inc. of King ofPrussia, Pa., or Cytec Industries Inc. of Woodland Park, N.J. In eachexample, the raw PEKK flake was subject to a heat treatment step toremove impurities from the PEKK flake. As described above, thetemperature was ramped up to 200° C. over a one-hour period. Thetemperature was then held at 200° C. for about six hours. Thetemperature was then ramped up a second time to 225° C. The temperaturewas then held at 225° C. for a minimum of one hour. The temperature wasthen ramped up a third time to 250° C. and held for at least one hour.

After heat treatment, the PEKK flake was milled by jet milling resultingin a plurality of particles ranging in size from less than 10 μm toabout 200 μm. The milling process was performed using 3000 lbs. lots ofPEKK flake. After milling air classification was used to separate fineparticles from the milled lots. The classification was set to removeparticles having a diameter of 30 μm or less from each lot. This isreferred to as fines material, whereas the remainder of the milled lotmay be referred to a non-fines material. In some embodiments of thepresent invention, the fines may be added back into the composition.

In each of the lots shown in Table 1, the PEKK powder forming the basisof the Cake B was mixed with carbon fiber when the PEKK powder was inits virgin form. The siloxane was added after two SLS builds forming theCake B. The virgin powder and carbon fiber were mixed in a ZeppelinFM-200 mixer. A first fraction of virgin powder was added and a secondfraction of carbon fiber was added. In each of the examples, the carbonfiber was 15% by weight of the resultant powder composition and thepercentage of virgin PEKK powder was 85% by weight. The virgin PEKKpowder had a mean size greater than 30 μm and more specifically a meansize between 40 μm and 70 μm.

In order to form the Cake B PEKK powder, the virgin PEKK powder andcarbon fiber mixture was used in two prior separate SLS builds to formthe Cake B powder. The bed temperature for each build was determinedpursuant to U.S. Pat. No. 10,112,342 to Hexcel Corp. for a Method ofAnalytically Determining SLS Bed Temperatures, the disclosure of whichis hereby incorporated by reference. The bed temperature for each SLSbuild is between the melting temperature T_(M) of the material and 20°C. below the T_(M). In reference to FIG. 5, DSC data for each powdertype is shown and the associated melting temperature. Virgin powderexhibits two melting points. The upper melting point is used todetermine the bed temperature. In each case, the powder is exposed tothe bed temperature for at least several hours during the SLS build.After the build, the sintered parts are removed from the bed andunsintered powder, now referred to as Cake A, is sieved and prepared forsubsequent SLS builds. After the second build, the PEKK powder isreferred to as Cake B. It includes about 85% PEKK by weight and about15% by carbon by weight.

The powder compositions of Example 2 and Example 3 were prepared fromthe Cake B powder obtained from the methods above. In preparing theExample 2, 99 lbs. of Cake B was obtained and then separated into thirtythree individual three pound bags using a scale. One pound of siloxaneresin was obtained. In these examples DOWSIL 4-7018 Siloxane resin wasused, however, the present invention is not limited in this regard andother sources of siloxane may be employed. The components were mixedusing a high shear mixer. In these tests, a Robot Coupe® kitchen mixerwas used. 1.5 lbs of Cake B was added to the bowl of the mixer. Then13.75 grams of the siloxane resin was added on top of the Cake B. Anadditional 1.5 lbs. of Cake B was added to the mixing bowl. The materialwas then subjected to four mixing times of thirty seconds. The mixedmaterial was then deposited into a lined drum and the steps wererepeated until all of the bags were mixed. The hand drum wassubsequently hand mixed prior to the SLS process. Example 3 was preparedin a similar manner with the exception that 98 lbs. of Cake B PEKKpowder was used and two pounds of siloxane resin was used. It should beunderstood to a person of ordinary skill in the art and familiar withthis disclosure that the present invention is not limited in thisregard, and that the mixing procedure may be scaled or performed in adifferent manner. For example, the components may be mixed in aproduction size high-shear mixer.

The test results below were performed with siloxane resin available fromDow Corning® under the name 4-7081 resin modifier. The siloxane isprovided as a plurality of dry particles having an average particle sizeof 45 μm. Thus, the average size of the siloxane powder is similar tothe average size of the PEKK powder, thereby facilitating printing.

In order to test the powder compositions in the SLS process an SLS printjob was performed using each powder composition to print testingspecimens in accordance with ASTM D638 (tensile) and ASTM D6272(flexural). ASTM D638 is a common plastic strength specification andcovers the tensile properties of unreinforced and reinforced plastics.The test method uses standard “dumbbell” or “dogbone” shaped specimensthat are tested on tensile testing machine. For each powder compositionat least five test specimens were manufactured in the in-planedirection, also referred to as the x-direction. Each of the testspecimens were tested pursuant to ASTM D638 and the results for eachdirection for each powder were averaged.

ASTM D6272 is a common plastic strength specification and covers theflexural properties of unreinforced and reinforced plastics byfour-point bending. The test method uses standard rectangular shapedspecimens that are tested. For each powder composition at least fivetest specimens were manufactured in the in-plane direction, alsoreferred to as the x-direction, and at least five test specimens weremanufactured in the out-of-plane direction or the z-direction. Each ofthe test specimens were tested pursuant to ASTM D6272 and the resultsfor each direction for each powder were averaged.

For each tested powder composition, the SLS builds were performed in aP800 machine. The process chamber temperature for each powdercomposition was determined in accordance with an analytical method fordetermining the bed temperature of an SLS machine. The laser power, orexposure, for each qualification was 6 W. The laser power was determinedin accordance with U.S. patent application Ser. No. 15/872,496 filed onJan. 16, 2018 by Hexcel Corporation and titled “Method for AnalyticallyDetermining Laser Power for Laser Sintering.”

The qualification builds using the 1% siloxane and the 2% siloxaneconsistently satisfied the acceptance criteria for the buildrequirements relative to the Cake B with 0% siloxane. The partsunexpectedly did not include any evidence of tiger striping commonlyappearing in Cake B SLS without siloxane. In addition, the powder with1% and 2% siloxane showed improved printing capability and part removal.All of these improvements benefit the economics of SLS of PEKK.Unexpectedly, the siloxane addition also improved the toughness of thematerial, which is a property that customers have been interested in.

TABLE 2 X-Direction 4-Point Flex Ex. Material Batch # n Strength (ksi)Modulus (ksi) Strain (%) 1 0% Siloxane 1734 5 21.3 898 2.35 2 1%Siloxane 1870 5 24.8 906 2.99 3 2% Siloxane 2116 5 20.4 729 3.05

Table 2 shows the testing results for the 4-point flexural test for thein-plane test specimens. Parts made from the powder composition with 1%siloxane showed a 16% increase in strength relative to the Example 1control. The modulus of elasticity remained essentially the same. Thestrain showed an increase of 27% over the Example 1 control. Theincrease in the strength and the strain are unexpected and considereddesirable in the market. The test coupons did not show any unacceptabledeviations in thickness or show any indication of tiger striping and alltest coupons were with −0.010″/+0.020″ of the specified dimension.During the build process there were no coating issues or powderapplications issues observed with the Example 2 powder. After the SLSprocess, the sintered parts were relatively easy to remove from the cakebed as opposed to sintered parts made from Cake B without siloxane.

Parts made from the powder composition with 2% siloxane showed a 4%decrease in strength relative to the Example 1 control. The modulus ofelasticity increased by 2% relative to the control. The strain showed anincrease of 30% over the Example 1 control. The increase in the strainis unexpected and considered desirable in the market. The test couponsprinted from 2% siloxane did not show any unwanted deviations inthickness or show any indication of tiger striping and all test couponswere with −0.010″/+0.020″ of the specified dimension. During the buildprocess there were no coating issues or powder applications issuesobserved with the Example 3 powder. After the SLS process, the sinteredparts were relatively easy to remove from the cake bed as opposed tosintered parts made from Cake B without siloxane.

TABLE 3 Z-Direction 4-Point Flex Ex. Material Batch n Strength (ksi)Modulus (ksi) Strain (%) 1 0% Siloxane 1734 5 17.9 812 2.16 2 1%Siloxane 1870 4 17.3 804 2.26 3 2% Siloxane 2116 5 9.17 542 1.72

Table 3 shows the testing results for the 4-point flexural test for theout-of-plane test specimens. Parts made from the powder composition with1% siloxane showed a 3% decrease in strength relative to the Example 1control. The modulus of elasticity remained essentially the same. Thestrain showed an increase of 5% over the Example 1 control. The testcoupons did not show any unacceptable deviations in thickness or showany indication of tiger striping and all test coupons were with−0.010″/+0.020″ of the specified dimension. During the build processthere were no coating issues or powder application issues observed withthe Example 2 powder. After the SLS process, the sintered parts wererelatively easy to remove from the cake bed as opposed to sintered partsmade from Cake B without siloxane.

Parts made from the powder composition with 2% siloxane showed a 49%decrease in strength relative to the Example 1 control in the z-axis.The modulus of elasticity decreased by 33% relative to the control. Thestrain showed decrease of 20% over the Example 1 control. The testcoupons printed from 2% siloxane did not show any unwanted deviations inthickness or show any indication of tiger striping and all test couponswere with −0.010″/+0.020″ of the specified dimension. During the buildprocess there were no coating issues or powder application issuesobserved with the Example 2 powder. After the SLS process, the sinteredparts were relatively easy to remove from the cake bed as opposed tosintered parts made from Cake B without siloxane. While the 2% siloxaneaddressed issues regarding powder application, build tolerances, andpart removal, a material reduction in the strength as compared to 0%siloxane was observed. Likewise, a significant reduction in the strainwas detected.

TABLE 4 X-Direction Tensile Ex. Material Batch n Strength (ksi) Modulus(ksi) Strain (%) 1 0% Siloxane 1734 10 15.9 1047 2.01 2 1% Siloxane 18705 14.8 944 2.23 3 2% Siloxane 2116 5 13.4 922 2.12

Table 4 shows the testing results for the tensile test for the in-planetest specimens. Parts made from the powder composition with 1% siloxaneshowed a 7% decrease in strength relative to the Example 1 control. Themodulus of elasticity decreased by 10%. The strain showed an increase of11% over the Example 1 control. The test coupons did not show anyunacceptable deviations in thickness or show any indication of tigerstriping and all test coupons were with −0.010″/+0.020″ of the specifieddimension. During the build process there were no coating issues orpowder applications issues observed with the Example 2 powder. After theSLS process, the sintered parts were relatively easy to remove from thecake bed as opposed to sintered parts made from Cake B without siloxane.

Parts made from the powder composition with 2% siloxane showed a 16%decrease in strength relative to the Example 1 control. The modulus ofelasticity decreased by 33% relative to the control. The strain showedan increase of 5% over the Example 1 control. The test coupons printedfrom 2% siloxane did not show any unacceptable deviations in thicknessor show any indication of tiger striping and all test coupons were with−0.010″/+0.020″ of the specified dimension. During the build processthere were no coating issues or powder applications issues observed withthe Example 2 powder. After the SLS process, the sintered parts wererelatively easy to remove from the cake bed as opposed to sintered partsmade from Cake B without siloxane.

In reference to FIGS. 3 and 4, images captured from a scanning electronmicroscope (SEM) of the printed PEKK coupons are shown. In FIG. 3, animage of a coupon printed from Example 1 powder is shown (no siloxane).Printed PEKK typically has a porosity of about 2% or less. The gaps inthe printed Cake B are visible in FIG. 3. In reference to FIG. 4, inimage of a coupon printed form Example 2 powder is shown (1% siloxane).The siloxane is shown in the picture filling in the gaps. The additionof the siloxane in the gaps services to enhance the strain of theexamples 2 and 3, and particularly as it relates to Example 2 having 1%siloxane. The carbon fiber is also visible.

While the siloxane is first added to Cake B in the test samples, thepresent invention is not limited in that regard. For example, siloxaneresin may be added to Cake A or to virgin powder. Likewise, while thepresent disclosure includes test examples that included 15% carbonfiber, the present invention is not limited in that regard. A person ofskill in the art and familiar with this disclosure will understand thatthe siloxane may be used to enhance the building of recycled PEKKwithout carbon fiber.

While the present teachings have been described above in terms ofspecific embodiments, it is to be understood that they are not limitedto those disclosed embodiments. Many modifications and other embodimentswill come to mind to those skilled in the art to which this pertains,and which are intended to be and are covered by both this disclosure andthe appended claims. It is intended that the scope of the presentteachings should be determined by proper interpretation and constructionof the appended claims and their legal equivalents, as understood bythose of skill in the art relying upon the disclosure in thisspecification and the attached drawings.

What is claimed is:
 1. A powder composition suitable for use inselective laser sintering for printing a three-dimensional object, thepowder composition comprising: a first fraction comprising apolyetherketoneketone (PEKK) powder having a plurality of particles, theplurality of particles having a mean diameter between 30 μm to 90 μm; asecond fraction comprising a siloxane powder having a plurality ofparticles.
 2. The powder composition of claim 1 further comprising: athird fraction comprising carbon fiber, the third fraction is between 5%and 25% of the composition by weight.
 3. The powder composition of claim2, wherein the PEKK powder has been previously used in a first SLS buildhaving a bed temperature greater than 250° C.
 4. The powder compositionof claim 3, wherein the PEKK powder has been previously used in a secondSLS build having a bed temperature greater than 250° C.
 5. The powdercomposition of claim 4 wherein the second fraction is between 0.0% and5.0% of the composition by weight.
 6. The powder composition of claim 5wherein the second fraction is between 0.5% and 2.5% of the compositionby weight.
 7. The powder composition of claim 6 wherein the secondfraction is between 0.5% and 1.5% of the composition by weight.
 8. Thepowder composition of claim 6 wherein the third fraction is 15% of thecomposition by weight.
 9. The powder composition of claim 6 wherein theplurality of siloxane particles having a mean diameter between 30 μm to60 μm.
 10. The composition of claim 9 wherein the plurality of siloxaneparticles having a mean diameter between 40 μm to 50 μm.
 11. A methodfor a layer-wise manufacturing of a three-dimensional object from thepowder composition according to claim 1, comprising the steps of:applying a layer of a powder composition on a bed of a laser sinteringmachine, the powder composition is the powder of claim 1; solidifyingselected points of the applied layer of powder by irradiation;successively repeating the step of applying the powder and the step ofsolidifying the applied layer of recycled powder until all crosssections of a three-dimensional object are solidified.
 12. The method ofclaim 11, wherein the object formed has a strain to failure of at least2.0%.
 13. The method of claim 11, wherein the object formed has a strainto failure greater than a strain to failure of an object made from aPEKK powder composition excluding siloxane powder.
 14. A method ofpreparing a powder composition suitable for use in laser sintering forprinting a three-dimensional object, the method including the steps of:providing a polyetherketoneketone (PEKK) powder having a plurality ofparticles; providing a siloxane powder having a plurality of particles;mixing the PEKK powder with the siloxane powder to obtain a powdercomposition suitable for use in selective laser sintering.
 15. Themethod of claim 14 wherein the mixing of the PEKK powder and thesiloxane powder is performed under dry conditions.
 16. The method ofclaim 14, further comprising the step of: providing a plurality ofcarbon fibers; mixing the carbon fibers into the powder composition. 17.The method of claim 16, wherein the PEKK powder has been previously usedin a first SLS build having a bed temperature greater than 250° C. 18.The method of claim 17, wherein the PEKK powder has been previously usedin a second SLS build having a bed temperature greater than 250° C. 19.The method of claim 18, wherein the siloxane powder is between 0.0% and5.0% of the composition by weight.
 20. The powder composition of claim 5wherein the second fraction is between 0.5% and 2.5% of the compositionby weight.